1. Field of the Invention
This invention relates to a process for producing tungsten monocarbide (WC) and more particularly to a process involving reacting a molten composition comprising an alkali metal halide and an oxygen compound of tungsten with solid carbonaceous reactants.
2. Description of the Related Art
U.S. Pat. No. 4,489,044 (Gomes et al.) discloses a method of producing tungsten monocarbide. A melt containing an alkali metal halide and an oxygen compound of tungsten is sparged with a gaseous hydrocarbon, particularly methane. The preferred alkali metal halide is sodium chloride, but the alkali metal halide may also consist of fluorides or bromides of sodium, potassium or lithium. Additionally mixed metal compounds such as NaAlF or KAlF may also be used. The oxygen compound of tungsten is wolframite, (Fe, Mn) WO.sub.4, or scheelite, CaWO.sub.4. The gaseous hydrocarbon is preferably methane in the form of natural gas, but other gaseous hydrocarbons can also be used such as ethane, acetylene, propane or butane as well as mixtures of the hydrocarbon gas or gases with H.sub.2 or CO. The Gomes et al. patent discloses that reactants such as powdered charcoal, coke breeze or calcium carbide may be added to the sodium chloride melt to facilitate reduction during sparging. Finally, the patent teaches that reduction and crystal growth may be assisted by the addition of small amounts of alkali metal borates, carbonates, fluorides or hydroxides.
The sparging is carried out in a conventional refractory crucible made from materials such as graphite or ceramics such as alumina, magnesia, zircon or silicon carbide. Refractory metal alloys such as Inconel are particularly effective. The sparging takes place at a temperature between about 900.degree. to 1100.degree. C. The flow rate of the gaseous hydrocarbon is between about 4 to 12 liters per minute per liter of melt for a period of about 3 to 8 hours.
The tungsten monocarbide produced by the method disclosed in the Gomes et al. patent has a microstructure in the form of platelets and twinned crystals. The crystals exhibit no major growth along the Z-axis. This morphology results in tungsten monocarbide that is not the norm of the cemented carbide industry and the crystals lack the strength necessary to make acceptable cemented carbide products.
The tungsten monocarbide produced by the Gomes et al. method also results in the carbon content being in the range of generally 6.08-6.10% by weight of the total WC. The correct stoichiometric amount of carbon in WC is 6.13% by weight. Thus the Gomes et al. method results in WC that is under-carburized and subsequent processing is required to increase the carbon content before the WC can be commercially utilized. The grain size of the WC produced by the Gomes et al. method is quite small, with more than 50% of the grains being smaller than 15 micrometers in average diameter.
It is an object of the present invention to produce tungsten monocarbide having a preferred crystal morphology. The crystals produced by the process of the present invention show greater growth along the Z axis, thus producing more blocky or thick bladed, and some equant, forms.
It is a further object of the present invention to produce crystals of tungsten monocarbide that contain carbon in the stoichiometrically correct ratio of 6.13% carbon by weight directly from a melt of an alkali metal halide and an oxygen compound of tungsten.
It is a feature of the present invention that tungsten monocarbide is produced from a melt of an alkali metal halide and an oxygen compound of tungsten by the addition of solid carbonaceous material and without the use of sparging.
It is an advantage of the present invention that tungsten monocarbide is produced having a large percentage of relatively coarse crystals that is acceptable for making sintered or cemented carbide products. The product also has application beyond cutting tools and wear parts, such as in hard facing welding rods and diamond bonding matrix powders.